The ability to respond to changes in the extracellular environment is fundamental to all cells. The growth of mammalian cells is controlled in part by extracellular cytokines which bind to integral membrane receptors that in turn regulate the activity of intracellular protein kinases. In Saccharomyces cerevisiae, the response to changes in extracellular glucose concentration shares many similarities with mammalian cytokine signaling pathways. The glucose sensor proteins encoded by SNF3 and RGT2 are integral membrane proteins that bind extracellular glucose and initiate a signaling pathway that ultimately results in changes in gene expression. The Snf1 serine/threonine protein kinase plays an essential role in signaling glucose limitation, however the mechanism by which the glucose sensors regulate the activity of the Snf1 kinase has yet to be elucidated. Studies of a yeast protein called Std1 provide a plausible and direct connection between the glucose sensors and the Snf1 kinase. A two-hybrid screen for Std1 interacting proteins identified the C-terminal hydrophilic domains of the glucose sensors, domains that have been shown essential for glucose signaling. Earlier studies have shown that Std1 physically interacted with Snf1 and that increased gene dosage of STD1 suppressed a mutation in the Snf1 kinase complex. Additional data document genetic interactions between the STD1 gene (and its homologue MTH1) and the genes encoding the glucose sensors that affect gene expression in response to changes in extracellular glucose levels. Moreover, interactions between Std1/Mth1 and glucose sensors regulate the activity of the Snf1 kinase. Since Std1 can interact with both the glucose sensors and the Snf1 kinase, the data support a model whereby the Std1 protein acts as a signaling intermediate between the membrane receptors and the intracellular protein kinase. The long term goal of these studies is to identify all of the proteins required for glucose signal transduction and to determine the sequence of their interactions and their mechanism of regulation. Three specific aims are proposed for this grant period: 1) Assess the biological significance of the interaction between the Std1 and Mth1 proteins with the glucose sensors; 2) Identify additional components of the Snf1 kinase signaling pathway; 3) Identify additional proteins that act in concert with Std1 and Mth1 to regulate gene expression.